Accurate measurement of very low fluid flows, in particular gas flows is a requirement of many industrial processes such as leak detection. Leak detection is used to determine the integrity of pressure vessels, containers and the like which contain or exclude fluids. Due to manufacturing technology improvements, very rapid economical testing of these vessels is required as a quality control measure to assure required vessel integrity in future performance. Such testing requires accurate measurement of very low leakage flow rates in the range of 0.01 to 10.00 standard cubic centimeters per minute (sccm) at a wide range of pressures while being subjected to harsh industrial environmental conditions. Many well known techniques that are utilized for this purpose are too costly, require unacceptably long testing times or require fluid temperature compensation which can introduce measurement inaccuracies.
In order to detect a leak within a containment vessel, a mass flow sensor may be connected between the pressurized containment vessel and a leaktight second pressurized vessel. Since the vessels are initially at equal pressure, a leak within the containment vessel reduces the pressure within the vessel causing a flow of gas into that vessel from the leaktight second vessel. Accordingly, the flow of gas from the leaktight vessel into the containment vessel would cause a gas mass flow rate through the mass flow sensor connecting the vessels which is proportional to the mass flow rate of the leaking gas.
A deposited thin film type mass flow sensor is best suited for fast, accurate measurement of these very low gas flows. Such deposited thin film type mass flow sensors are described in U.S. Pat. No. 4,651,564 as incorporated herein by reference, or as manufactured by Honeywell, Inc. as the MICRO Switch Model AWM200V MICROBRIDGE. A deposited thin film sensor includes a substrate which has a temperature sensing element disposed on both sides of a heating element. As a gas, referred to as the measured gas, flows across the substrate, it contacts the first temperature sensing element. The measured gas flow carries heat away from the first temperature sensing element, thereby reducing the temperature and electrical resistance of the temperature sensing element. Before the measured gas reaches the second temperature sensing element, the gas is warmed as it crosses a heating element which transfers heat to the measured gas. This heat transfer is sensed as the measured gas flows across the second temperature sensing element. The rate of heat transfer to the measured ga is a function of the mass flow rate of the measured ga and the temperature differential of the measured gas with respect to the heated film. The excess temperature of the heating element is controlled at a selected value. Accordingly, the temperature variation between the temperature sensing elements causes a resistance differential which is proportional to the mass flow rate of the measured gas. The heating and sensing elements referred to above are low mass, unsupported deposited thin films. The measured gas passes over and under each film assuring optimum heat transfer and rapid response to mass flow changes in the measured gas.
Heated deposited thin film sensor have deficiencies which limit their use in industrial leak testing. A deposited thin film type mass flow sensor is typically enclosed in a housing which directs the measured gas flow across the sensing and heating elements. A thin film sensor is fragile in that it has an operating pressure limit of 5 psi differential between the interior and the exterior of this protective housing which surrounds the sensor. The sensor, when enclosed only by the protective housing, could not operate under actual leak test pressures which may range from 25" Hg vacuum (about 200 mBar) to greater than 160 psig (11 Bar).
A second deficiency of heated film type mass flow sensors is flow rate measurement inaccuracy that is caused by variations in the ambient temperature or the temperature of the measured gas. Ambient air temperature variations cause the temperature sensing circuitry associated with the sensor to operate at a different temperature. To achieve the required flow sensitivity, precision operational amplifiers are utilized with closed loop gains of 1,000 to 10,000. At these gain levels, a temperature change causes a subsequent change in the nominal value of the resistors of the temperature sensing circuitry introducing measurement errors which are significant in low mass flow applications.
Undesirable changes in the sensitivity of the mass flow sensor also result when changes in the temperature of the measured gas cause inaccurate flow measurements. As the measured gas crosses over the substrate, the fluctuating temperature of the gas causes a variable rate of heat transfer between the sensing elements of the flow sensor because of the variable temperature of the heating element. To optimize measurement accuracy, the heating element of the mass flow sensor is maintained at a constant temperature differential above the ambient temperature which is sensed by the heater control circuitry associated with the sensor. The constant temperature of the heating element reduces, but does not eliminate the effects of variations in the measured gas' properties which may affect the resistances of the heating and sensing elements. A differential between the measured gas temperature and the ambient temperature may cause undesirable changes in both the initial offset, "zero", and in the sensitivity of the mass flow reading.
A third disadvantage in measuring very low gas flows with deposited thin film sensors is the tendency of the sensor to respond to random, short duration flow disturbances which are often present in industrial leak testing. These disturbances are normally dampened by electronic means that often require costly electronic filters which can introduce significant inaccuracies. This is particularly severe if an average flow rate is to be measured because the electrical output of the sensor for large fluctuations in flow input is generally asymmetrical or non-linear so that applying electronic dampening after the non-linearity causes a spurious change in the average reading for large higher frequency flow fluctuations.
In industrial leak testing applications, there has been a need for mass flow sensors which may be used to measure very low gas flow rates at a wide range of operating pressures and at varying ambient and measured gas temperatures. A mass flow sensor which correctly averages very low gas flow rates without responding to random, short duration flow disturbances is required to assure accurate mass flow rate measurements.